Certain industrial processes produce a saltwater waste (contaminated saltwater) while also requiring a lower salinity make-up stream. Examples include, but are not limited to:                Mineral extraction: water and chemicals are mixed with mined rock to extract a desired mineral such as copper or gold from the rock. Saline waste water, known as “tailings”, is produced as a result. The tailings need to be discharged while make-up water of lower salinity may be required to maintain production. In some cases, it may not be possible to recycle tailings water due to its high salinity. Lower salinity make-up water is required in order to prevent corrosion in the process plant or to ensure effectiveness of mineral extraction. It would be beneficial to desalinate tailings water for re-use while also concentrating and reducing the volume of the final discharge, thereby reducing discharge costs, environmental impacts, and freshwater extraction from other sources.        Oil sands extraction: oil sands may be converted to a vendible product by exemplar mining, separation and cracking, or by exemplar steam assisted gravity drainage processes. Both of these processes are commonly practiced in Canadian oil sands operations and produce a saline waste stream while also requiring lower salinity make-up water. It would be beneficial to desalinate the waste saline stream for re-use. The primary requirement is to remove scaling salts such as calcium and magnesium and corroding salts such as chlorides whereas hydrocarbons present in the water may remain.        Enhanced oil recovery: water is injected into hydrocarbon formations to displace and recover addition hydrocarbons. The injected water may be mixed with caustic, surfactants, and proprietary polymers that further enhance recovery and prevent formation plugging. Practitioners of enhanced oil recovery have found it preferable to inject water with a net salt concentration between 2,000 and 8,000 parts per million (ppm). More concentrated saline water, such as seawater with a salt concentration of 35,000 ppm reduces oil recovery. Many in industry believe the increased salt concentration reduces calcium ion exchange in the formation clays and prevents release of hydrocarbon molecules. Water with too low a salt concentration may also be detrimental, for example reverse osmosis permeates with a concentration of 400 ppm. Freshwater has been found to swell the formation clays and hence impede hydrocarbon movement and reduce recovery. Therefore, industrial experience has shown that water with a salt concentration of 2,000 to 8,000 ppm is preferred. Said water would preferably be rich in monovalent ions such as sodium and chloride but weak in divalent ions such as calcium and sulfates in order to assist in calcium ion exchange in the formation clays.        
After being injected, saltier water is often reproduced with the oil. Salinity of the water increases due to the presence of salts in the formation. The salinity of produced water can be highly variable, for example from 500 to 200,000 ppm. At present, produced water is commonly disposed and seawater desalinated for injection. The produced water may be re-used and additional chemicals such as surfactants and polymer added. It is known however, that in order to be effective, more polymer needs to be added for higher salinity waters resulting in increased operational costs. If the produced water is desalinated before polymer is added, then less polymer can be used. Polymers costs an average of $20-30M per year per platform, and savings could be in the $15M per year range by desalting the produced water first.
It would be beneficial to desalinate the produced contaminated saltwater waste, thereby reducing polymer addition requirements, waste water discharge, make-up water requirements, and leaving some of the chemicals added present in the desalinated produced water so as to reduce future input chemical input requirements.
Desalination of seawater and brackish water is commonly practiced. Desalination of industrial wastewater is also practiced, yet presents unique challenges due to the presence of compounds, such as hydrocarbons or chemicals not found in seawater or brackish water. Pre-treatment may be employed to remove said compounds; however, pre-treatment increases the cost of desalination. A brief review of the most commonly practiced desalination processes are as follows:                1. Reverse osmosis (“RO”): water is forced through an osmotic membrane that rejects salts and allows water flux under pressures in excess of the osmotic pressure. RO is presently the most widely practiced seawater desalination process. RO has challenges with industrial waste saltwater due to deleterious compounds, such as hydrocarbons that permanently foul the membrane, which cannot be adequately or economically removed with pre-treatment. Reverse osmosis also reaches osmotic pressure limits with saltwater reject waste stream (“brine”) concentrations at 80,000 ppm, therefore making it unsuited for high salinity waters and requiring additional brine treatment for inland operations. Reverse osmosis is currently not a fit for many industrial processes due to extensive pre-treatment requirements to reduce hydrocarbon and organic content to below 10 ppm levels, in addition to its product water being too pure for exemplar enhanced oil recovery processes.        2. Thermal: water is evaporated and condensed, at times in multiple effects in order to recycle the latent heat of condensation. The condensed freshwater is used as a product and the remaining brine discharged. Thermal process may include multiple effect desalination (MED), multi-stage flash (MSF), and vapour compression (VC). Thermal processes are more tolerant to deleterious substances such as hydrocarbons, produce an almost pure distillate, and can achieve very high brine concentrations including the potential for solids formation in exemplar VC crystallizers. However, thermal processes can be expensive and environmentally intensive due to their high energy requirement and costly materials of construction such as alloyed steels and titanium. Thermal process are the most common industrial waste saltwater desalination processes currently practiced but there is a need for less expensive and environmentally intensive processes. In addition, product water from thermal processes is pure distillate requiring blending with saltwater for processes that cannot tolerate pure water, such as enhanced oil recovery.        3. Electrochemical:                    a. ion exchange (“IX”) in packed resin beds where the IX resins exchange scaling ions such as calcium for sodium. IX requires frequent chemical inputs such as sodium chloride and hydrochloric acid in order to regenerate the resins—for example: remove the calcium from the resin and replace it with sodium. IX resin regeneration often produces an acid waste stream that must be managed, adding to complexity and cost. IX processes have proven to be more tolerant to hydrocarbons than reverse osmosis, and therefore have found application in oil and gas waste water desalination and softening. That said, the saltwater is not desalinated, it is softened with divalent ions replaced with less problematic monovalent ions, as a result chlorides are not removed and therefore the corrosive potential of the water is not reduced.            b. electrodialysis transfers salt ions across ion exchange membranes under the application of a galvanic potential. The galvanic potential is supplied as a voltage generated at an anode and cathode. Ion exchange membranes offer the advantage that they do not require regeneration, thereby reducing the need for chemical inputs over IX processes. Membrane inorganic scaling can be managed through polarity reversal (electrodialysis reversal—EDR) and fouling managed through periodic flushes or dilute acid washes. Unlike reverse osmosis, the output product water concentration from electrodialysis can be adjusted by adjusting the voltage applied to the stack.                        
Traditional electrodialysis stacks consist of two chambers—a diluent and concentrate. Salt ions are transferred from the diluent to the concentrate under the direct current electric field applied at the electrodes. The concentration factor across any single membrane has limits, which is expressed as the ratio of concentrate to diluent salt mass. A practical concentration factor of five to ten is common. For example, transferring ions from a diluent with a concentration of 2000 ppm to a concentrate with a concentration of 10,000 to 20,000 ppm. It is not to transfer ions from a diluent of 2000 ppm to a concentrated of 200,000 ppm.
Concentration polarization at the membrane surface increases with concentration factor, thereby limiting current density. In addition, back diffusion across the membrane increases with concentration factor, thereby reducing current efficiency. Reducing the concentration factor across a single membrane will generally increase the maximum allowable current density and also increase current efficiency. Concentration factors can be limited with two chamber stacks by use of external staging of the stacks. For example, a first stack's diluent and concentrate concentrations being respectively low (2,000 ppm) and moderate (20,000 ppm), and the second stack's diluent and concentrate concentrations being moderate (20,000 ppm) and high (200,000 ppm). By inserting the moderate concentration circuit, the concentration factor across the ion exchange membranes is reduced. However this requires multiple stacks with increased footprint for their multiple frames and process pipework.
It would be beneficial to devise a process that has the advantages of electrodialysis in terms of increased fouling tolerance, descaling through ionic current reversal, and ability to tune the output product water concentration, but also enables a high concentration difference in a single electrodialysis stack allowing more compact desalination and production of a highly concentrated low volume discharge saltwater. At increased concentration, however, the potential for precipitation and crystallization internal to the stack increases. Even with electrodialysis reversal a stack may operate well on highly scaling waters for 2-3 weeks but eventually precipitates form internal to the stack, blocking flow channels. It would therefore be beneficial to devise a control process and scheme that senses the on-set of membrane scaling and internal stack precipitation, and takes action to prevent its propagation.
Certain saltwater sources, such as inland brackish water, can have increased concentrations of “hard” ions such as calcium and magnesium relative to seawater. Said hard ions can present inorganic scaling issues on desalination mass and heat transfer surfaces; respective examples include RO membranes or MED heat exchange surfaces. Scaling is mitigated by limiting recovery thereby reducing the scaling ion concentration present at the mass or heat transfer surface. Recovery is defined as the volumetric flow rate of desalinated water production relative to feed water input. Reducing recovery reduces the concentration of the ions in the brine reject, thereby reducing scaling potential of the mass or heat transfer surfaces exposed to the highest concentration saltwater. However, reducing recovery detrimentally limits the production of desalinated water.
Removing hard scaling ions from the plant feedwater enables desalination plant operation at a higher brine reject concentration and therefore a higher recovery, resulting in increased desalinated water production. Hard ions such as calcium and magnesium may be removed from the desalination plant feed water via conventional methods known to those skilled in the art such as lime softening or cation ion exchange (CIX). Both lime softening and cation ion exchange systems require the input of chemicals: such as sodium carbonate, regeneration acid or base, or sodium chloride. Chemical consumption and waste generation can be quite high for lime softening and cation exchange systems—in the order of many truck loads per day for an exemplar 10,000 m3/day desalination system. The addition of chemicals presents ongoing operational costs along with increased safety and hazard risks. It would therefore be beneficial to devise a system that removes scaling ions from desalination plant feed water without the need for chemical addition.
In EDR scaling ions, such as calcium and magnesium may pass through the electrode membrane and into electrolyte chambers. The scaling ions may precipitate and causes scaling in the electrode chamber which cannot be easily remove without shutting down operation of the EDR. It would therefore be beneficial to devise an EDR system that prevents or reduces scaling ions from passing into the electrolyte chamber.
Waters contaminated with relatively low levels of salt concentration can still be unusable or hazardous to the environment. For example, mines use freshwater and discharge tailings into ponds. Tailings water is commonly 99.8% freshwater by mass, but unusable due to low level salts, for example 0.1 to 0.2% by mass. Commonly encountered salts include calcium, sulfates, chlorides, carbonates, heavy metals, iron, selenium, and arsenic. Run-off from exposed rock can also contain low level, but hazardous salt concentrations. For example, in the case of one form of “acid rock drainage,” iron leaching from exposed rock can initiate a reaction where acid is formed, with the acidity increasing the rate of iron leach and propagating acidification, thereby causing run-off water to become hazardous.
Acidic streams near abandoned mines are commonly treated with lime or caustic addition to neutralize the acidity and precipitate out metals. This process requires chemical inputs that may be caustic, which present cost, transport, and handling challenges. Mine operators are starting to practice reverse osmosis to remediate a portion of their tailings. Reverse osmosis produces an almost pure permeate freshwater by pressuring saltwater through a semi-permeable membrane, also resulting in the production of higher salinity brine. Recovery of the reverse osmosis system, defined as produced freshwater relative to input saltwater volume, is often limited by the concentration of the scaling ions in the higher salinity brine. The recovery of the reverse osmosis system must be limited to the 4 to 5% salt mass range, often due to the scaling salts listed above. This leaves a large volume of un-treated brine behind which still consists of 95% freshwater.
Un-treated reverse osmosis brine may by disposed by deep well injection if such geology and regulatory framework exists. Other commonly practiced brine management options include:                Return the brine to the tailings impoundment: this does not remove salts from the water balance and leads to an eventual increase in the salt concentration of the tailings impoundment, which is a problem if the impoundment is also the reverse osmosis plant feed source. With time, the concentration of the tailings will rise and further limit the recovery of the reverse osmosis system.        Zero liquid discharge in a mechanical or thermal vapour compression crystallizer: this removes the salt from the water balance, but is a capital and energy intensive process. Due to the often low recovery of reverse osmosis system a high capacity crystallizer is required, resulting in high total costs.        
When a first stage process, such as reverse osmosis, is hybridized with a second stage zero liquid discharge process, such as a crystallizer, it is beneficial to minimize the volume of the saltwater sent to the more costly second stage. This will minimize the capacity of the more costly second stage and can be achieved by maximizing the concentration of the saltwater output from said first stage. For example, doubling the first stage saltwater output concentration from 4% salt to 8% salt will halve the size of the second stage.
Electrochemical processes such as electrodialysis reversal (EDR) move salts across ion exchange membranes into a more concentrated saline solution. EDR is known for its ability to operate at higher reject concentration than reverse osmosis, due to two primary reasons:                1. Ionic de-scaling of membranes through polarity reversal, which periodically “back-flushes” salt flux through the membranes and de-scales them in the process. It is not possible to back-flush reverse osmosis systems.        2. Unlike reverse osmosis, EDR output concentration is not limited by osmotic and hydraulic pressure barriers. Reverse osmosis systems have a peak pressure rating (commonly 1200 psi) and freshwater will not be produced unless the hydraulic pressure exceeds the osmotic pressure (commonly limited to 8% salt mass so as to not exceed 1200 psi).        
It would be beneficial to devise an improved two stage process for desalinating low salinity water where the first stage increasing the concentration of the output saltwater and as a result beneficially reduces the capacity of the second stage solution concentrating desalination system.